This invention relates to a small-sized zoom lens system lending itself well fit for electronic still, video and other cameras using solid state image sensors, etc. as the image pickup devices.
In recent years, radical reductions in the size and weight of electronic still and video cameras have been in the making. Solid state image sensors are now decreasing in size from 1/2 in. or a diagonal length of ca. 8 mm to 1/3 in. or a diagonal length of ca. 6 mm and further to 1/4 in. or a diagonal length of ca. 4 mm, with an increasing demand for reductions in the size of corresponding lens systems. Conventional lens systems which are designed to achieve such size and weight reductions and belong in type to that of the invention to be described later are set forth in Japanese Provisional Patent Publications Nos. 2(1990)-236514 and 64 (1989)-91110, for instance. These systems make sure of sufficient back focal distances because of using prism half or quick return mirrors as means for splitting rays to a finder optical system. As described in Example 10 of the latter publication teaching the shortest total length, however, the total length is still as long as expressed by EQU Total length/f.sub.W &gt;4.46
wherein the total length is the distance from the first surface to the final surface of the lens system and f.sub.W fill is the focal length of the total system at the wide angle end. Thus, there is left much to be desired in connection with size reductions. When lens systems are scaled down according to a certain proportionality factor to accommodate to image pickup sensors of 1/3 in. or 1/4 in. in size, some problems arise in the course of processing lenses; for instance, the edge and intermediate thicknesses of convex and concave lenses become insufficient.